Arc welding consists of several processes that utilize an electric arc produced by electric current passing through an ionized gas as a source of heat to melt and join metals. Typically, the welding arc is formed between the piece being welded, or base metal, and an electrode. A "good" weld is commonly defined as a weld which fuses metals in a bond having strength to withstand any stresses which are expected to be encountered.
There are many causes of defects which can occur during the welding process, preventing this fusion from occurring and creating welds which will often not withstand stress. Specific types of weld defects which can occur during the arc welding process include a condition commonly referred to as spatter in which a considerable amount of spatter or filler metal is produced which does not enter the weld due to motion of the filler metal during the welding process. Loss of shielding gas, used to protect the arc and weld zones from air and provide desired arc characteristics in gas metal arc welding, can further result in weld defects.
Additionally, the condition known as arc blow can detrimentally affect the welding process. This condition primarily occurs when a change in direction of current flow as it enters the work and is conducted to the work lead, or when there exists an asymmetric arrangement of magnetic material around the arc, as in when welding is done near the end of ferromagnetic materials. Arc blow occurs as a result of magnetic disturbances surrounding the welding arc, resulting in the arc being directed away from the point of welding.
Weld contamination is another type of weld defect often occurring during welding. Weld contamination results when foreign materials or substances, such as brass, concrete, or plastic, commonly present in factory environments, are introduced into the welding material, weakening the weld subsequently formed. There are numerous other events which can occur during the welding process, adversely affecting the quality of the weld produced.
In some instances, weld defects, such as those described above, are identifiable to the ear of an experienced welder, although identification in this manner can be hampered when the welding is occurring in an environment having a significant amount of background noise, such as a factory setting. In automated welding systems, however, it is more difficult to identify and correct weld defects. In particular, in robotic welding systems, real-time analysis of weld quality is required so that the system can either correct the situation causing the defect, or in the alternative, provide notification to an operator that such a defect has occurred.